There are three principal methods of detecting the presence of carbon monoxide (CO) in air. The first method of detection uses a plug-in detector having a periodically-heated semi-conductor that exhibits a change in conductivity when CO is present. However, this type of detector requires AC-power, and ceases to function when electricity to the unit fails. The detector tends to be sensitive to changes in humidity, and is cross-sensitive to the presence of other combustible gases e.g. alcohols, including materials containing alcohols, examples of which include hairspray.
The second type of detector uses a translucent gel disk that darkens on prolonged exposure to CO. The change in translucency is detected by an infrared sensor within the unit. Detection tends to be less responsive than for other detectors, taking hours rather than minutes to recover after the ambient air has become free of CO. Consequently, it becomes necessary to remove the battery-sensor pack in order to silence the alarm that sounds when CO is detected. In addition, the gel tends to accumulate CO over a period of time, resulting in a tendency for false alarms after prolonged exposure to urban pollution.
The third type of detector uses a fuel-cell type electrochemical sensor. These detectors are battery-powered and are much more accurate and responsive to the presence of CO.
The electrolytic cell of an electrochemical sensor must have at least two electrodes. One electrode is the electrode that comes in contact with the gas that is to be detected, and is usually referred to as the sensing electrode. A second electrode is known as the counter electrode or auxiliary electrode. When the gas to be detected comes in contact with the sensing electrode, an oxidation reaction takes place at the sensing electrode, with a corresponding reduction reaction occurring at the counter electrode.
The potential of the sensing electrode must be sufficiently positive so that CO will be oxidized. However, the potential of the sensing electrode is subject to change, because the use of a fixed external voltage bias inter-relates the potential of the sensing electrode to the potential of the counter electrode. The potential of the counter electrode is unstable if the electrode material is not electrochemically reversible, i.e. the exchange current density is not high enough compared with the current passing through the cell. Consequently, it is possible that the potential of the sensing electrode will shift to a value where CO is not fully oxidised at the sensing electrode.
Thus, it can be important to have an electrode with a constant or almost constant potential throughout the reaction. Such an electrode is called the reference electrode and its main role is to stabilize the potential of the sensing electrode. In that event, the potential of the sensing electrode will remain relatively stable so that CO may be quantitatively oxidized.
Other detectors use only two electrodes. In such detectors, the reference electrode also serves as the counter electrode. Any current generated by the sensing electrode passes through this reference/counter electrode.
CO sensors using electrochemically reversible materials such as lead dioxide (PbO.sub.2) and silver as reference electrodes show high sensitivity to the presence of CO and a wide linear response range, from below 5 ppm to over 10% v/v. The sensors are robust and reliable, and may be used under demanding conditions e.g. analysing stack gases from industrial plants, monitoring toxic gas concentrations in omissions from a gas producing process and the like. However, such sensors generally require two or more hours for the background current to stabilize, because an initial O.sub.2 /H.sub.2 O redox coupling reaction controls the reference potential. The sensors have a moderate sensor life, generally of less than 2 years, and tend to be bulky in order to hold sufficient sulphuric acid solution required for operation of the sensor.
An example of a two-electrode sensor is described in U.S. Pat. No. 3,775,125 and examples of three-electrode sensors are described in U.S. Pat. Nos. 4,587,003, 5,284,566 and 5,338,429. In all of these sensors, a platinum/air/water electrode was used as reference electrode. However, such sensors have a number of disadvantages, including (a) high cost due to the use of precious metals e.g. platinum foils and wires, (b) the requirement of strict performance criteria in contact between electrodes and precious metals, and high failure rates due to poor contact, (c) leakage of electrolyte after long periods of operation, (d) costs of assembly of numerous parts of the sensor, and (e) large piece-to-piece variations in sensor output.